The invention relates to a lead frame used for semiconductor integrated circuits and its bonding method, and more particularly to a lead frame structure suitable for a thermal compression bonding in a tape carrier method used for fabrication processes of semiconductor integrated circuits.
A tape carrier method has widely been used for fabrication of semiconductor integrated circuits because the method is suitable for automatic bonding of lead frames. In the tape carrier bonding method, lead contacts having a conductivity are formed on a flexible film made of an insulator such as a polyimide based resin. Generally, such lead frames are directly bonded to bumps of semiconductor devices by use of a thermal compression method.
The tape carrier bonding method as prior arts will subsequently be described with reference to FIG. 1. A film 1 comprising a homogeneous flexible tape, which is made of an insulator material such as polyimide is prepared. The film 1 is provided at its opposite sides with a plurality of sprocket holes 2 at regular intervals and provided at its center portion with device holes 4. Subsequently, lead patterns are formed on the film 1 by using a method such as metal etching, or corrosion so that a top portion 3A of each of lead contacts 3 of the lead patterns protrudes into the device hole 4 thereby permitting the top portion 3A of the lead contacts 3 to be bonded to bumps of a semiconductor device 5 such as IC chips by using a thermal compression. The each lead contact also has an opposite end to the top portion 3A, which serves as a contact portion 3C by which a measurement of electric characteristics of the semiconductor device is accomplished by contacting a probe with the contact portion 3C.
The film 1 is provided at its opposite sides with a pluraity of the sprocket holes at regular intervals so that the sprocket holes may be engaged with tooth gears having the same pitch as the sprocket holes and automatically transmitted with the teeth gears. Such lead patterns are arranged on the film 1 at regular intervals thereby permitting automatic bonding which will subsequently be described with reference to FIG. 2.
A bonding process of the lead frame and the semiconductor deice is accomplished as follows. The top portion 3A of the lead contact 3 are arranged so as to overlay a bump on the semiconductor device 5, after which the top portion 3A is pressed down with a heating element 6 serving as a bonding tool so that the top portion 3A is bonded with the semiconductor device 5 through the bump 5A. Such prior arts are, however, engaged with following disadvantages. Such pressure welding with using the heating element 6 serving as the bonding tool forms a depressed area in the top portion of the lead frame thereby forming a sharp drop portion 3D in the lead frame, which is defined by an edge portion of the heating element 6 of the bonding tool. The sharp drop portion 3D of the lead frame is likely to cause cracks during or after the bonding process to the device. The lead frame may readily be cut due to extremely weakness in strength of the sharp drop portion 3D.